Abstract Management of inflammatory bowel disease (IBD) patients requires knowledge of disease status to inform treatment decisions. Commonly used biomarkers such as clinical disease activity indices, C reactive protein (CRP) and fecal calprotectin achieve suboptimal sensitivity and specificity for intestinal inflammation1. Data provided indicate that peripheral blood exosome (PBE) lipid composition distinguishes active IBD from normal patients (Fig. 1). We contend that PBE lipid compositions will provide clinicians with a highly sensitive and specific biomarker to assess disease activity without invasive testing. The need to identify subtle disease derives from the consensus conclusion that ?deep remission? should be the endpoint of therapy. Thus, monitoring of patients with intent to suppress subclinical inflammation has emerged as a treatment goal3. Exosomes are lipid-encased, subcellular (60-80 nm) structures released into the peripheral blood at from intestinal epithelial cells (IEC), activated leukocytes (e.g. neutrophils, macrophages, dendritic cells, etc.) and mesenchymal cells during inflammation3. We used an ultrahigh resolution Orbitrap mass spectrometer to analyze lipid compositions of PBEs from patients (>25/group). Principal component analysis (PCA) of PBE lipid intensities showed a wide separation of datapoints between active IBD and normal controls and a heatmap analysis of differential abundance revealed high within-group correlations and low between-group correlations suggesting that distinct lipids can be resolved that correlate with disease activity. In this two year project, we propose to collect plasma from 1) moderate-severe, 2) mildly-active and 3) quiescent Crohn?s disease (CD) patients as well as inflammatory (C. difficile-infected) and 4) normal controls. Using CD allows us to test whether PBE composition detects levels of mild (in many cases, subclinical) disease activity as this identifies patients not in deep remission, a goal of medical therapy. In Aim 1, PBE lipid-based classifiers will be developed to discriminate mildly-active from severely-active CD and controls. In Aim 2, PBE lipids will be examined longitudinally within individual patients before and after therapy to identify lipids correlate with clinical response. Such information will also be passed to Aim 1 to develop more robust classifiers. Testing patterns of PBE lipids in responsive and refractory patients is intended to improve the robustness of the classifiers. Studies in Aim 1 and 2 are greatly enhanced by the inclusion of a second clinical site (Baylor University) to provide samples to validate (or not) data from University of Kentucky. The long-term goal will be to justify studies to identify a ?exosomal lipid signature? panel that provide a novel set of biomarkers for discriminating levels of CD disease activity and informing treatment decisions. We post that the creation of a CD exosome lipid biomarker test will obviate the need for follow-up endoscopy in the majority of patients.